Solid-state image pickup device

ABSTRACT

A solid-state image pickup device  1  according to the present invention includes a semiconductor substrate  2  on which a pixel  20  composed of a photodiode  3  and a transistor is formed. The transistor comprising the pixel  20  is formed on the surface of the semiconductor substrate, a pn junction portion formed between high concentration regions of the photodiode  3  is provided within the semiconductor substrate  2  and a part of the pn junction portion of the photodiode  3  is extended to a lower portion of the transistor formed on the surface of the semiconductor substrate  2.  According to the present invention, there is provided a solid-state image pickup device in which a pixel size can be microminiaturized without lowering a saturated electric charge amount (Qs) and sensitivity.

The present application claims priority to Japanese Patent ApplicationJP2004-028353, filed in the Japanese Patent Office Feb. 4, 2003; theentire contents of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid-state image pickup device inwhich a pixel size can be microminiaturized without lowering a saturatedelectric charge amount (Qs) and sensitivity.

2. Description of the Related Art

A CMOS (complementary metal-oxide semiconductor) type solid-state imagepickup device is known as a solid-state image pickup device. This CMOStype solid-state image pickup device is composed of a plurality ofpixels arranged in a predetermined pattern in which one pixel iscomprised of a photodiode and a plurality of transistors, that is,so-called MOS (metal-oxide semiconductor) transistors. The photodiode isa photoelectric converting device for generating and accumulating signalelectric charges corresponding to a received amount of incident light.

FIG. 1 of the accompanying drawings is a schematic cross-sectional viewshowing an example of a related-art CMOS type solid-state image pickupdevice that is applied to an image sensor. FIG. 1 shows, in particular,a main portion of the pixel. As shown in FIG. 1, in this CMOS typesolid-state image pickup device 51, a pixel separation region 65 forseparating each pixel is formed on a first conductivity type, forexample, p type silicon semiconductor substrate 52. A photodiode 53 anda plurality of MOS transistors, that is, four MOS transistors of anelectric charge readout transistor 54, a reset transistor 55, anamplifying transistor 56 and a vertical selection transistor (not shown)are formed on each separated region and thereby a unit pixel 60 isconstructed. Then, a large number of pixels 60 are arrayed in atwo-dimensional matrix fashion (that is, in an XY matrix fashion).

The photodiode 53 is composed of a second conductivity type, that is, ann type semiconductor region 61 [n⁺ regions 61 a, 61 b] formed from thesurface of the p type semiconductor substrate with a predetermined depthby ion implantation and a p type semiconductor region (p⁺ region) 62with a high impurity concentration formed on the surface of the n typesemiconductor region 61.

Each of MOS transistors 54, 55 and 56 is constructed as follows. On thesurface of the p type semiconductor substrate 52, there are formed ntype semiconductor regions with a high impurity concentration, that is,n⁺ source-drain regions 57, 58 and 59 by ion implantation so as toadjoin the photodiode 53.

The electric charge readout transistor 54 is composed of an n⁺source-drain region 57, an n⁺ region 61 a with a high impurityconcentration on the surface side of the photodiode 53 and a gateelectrode 72 formed on the substrate 72 between the two regions 57 and61 a through a gate insulating film 71.

The reset transistor 55 is composed of n⁺ source-drain regions 57 and 58and a gate electrode 73 formed on the substrate 52 between the tworegions 57 and 58 through the gate insulating film 71. The n⁺source-drain region 57 is what might be called a floating diffusion (FD)region.

The amplifying transistor 56 is composed of n type source-drain regions58 and 59 and a gate electrode 74 formed on the two regions 58 and 59through the gate insulating film 71.

Although not shown, in a like manner, the vertical selection transistoris comprised of a pair of source-drain regions and a gate electrodeformed on the substrate 52 between the two regions through a gateinsulating film.

Circuit interconnections of the respective MOS transistors are similarto those which will be described later on and therefore need not bedescribed. The n type source-drain region 58 for connecting the resettransistor 55 and the amplifying transistor 56 of each pixel isconnected to a power supply interconnection 76 through a connectorconductor 75. Further, a multilayer interconnection 77 including thepower supply interconnection 76 is formed on the substrate 52 through aninterlayer insulator 78.

This CMOS type solid-state image pickup device 51 introduces light fromthe surface side of the semiconductor substrate 52 into the photeodiode53, it photoelectically converts incident light by the photodiode 53 andthen it accumulates signal electric charges corresponding to thereceived amount of incident light.

A cited patent reference 1 has proposed a solid-state image pickupdevice of an MOS type image sensor in which a photodiode, an electriccharge readout transistor, a reset transistor, an amplifying transistorand a vertical transfer transistor comprising the above-mentioned unitpixel are formed on the same plane of the same substrate (see citedpatent reference 1).

[Cited patent reference 1]: Official Gazette of Japanese Laid-openPatent Application No. 11-122532

Although the above-mentioned CMOS type solid-state image pickup device51 is microminiaturized in order to integrate a large number of pixels60 with a high integration degree, since a plurality of transistors suchas the photodiode 53 and the electric charge readout transistor isdisposed on the same plane of, in particular, each pixel region, eachpixel requires the area on the plane and it is unavoidable that the areaof one pixel is increased. For this reason, it becomes difficult tomicrominiaturize the pixel size. When the pixel size ismicrominiaturized, the area of the photodiode 53 is decreased andproblems arise, in which the saturated electric charge amount (Qs) islowered and in which sensitivity is lowered.

SUMMARY OF THE INVENTION

In view of the aforesaid aspect, it is an object of the presentinvention to provide a solid-state image pickup device in which a pixelsize can be microminiaturized without lowering a saturated electriccharge amount (Qs) and sensitivity.

According to an aspect of the present invention, there is provided asolid-state image pickup device which is comprised of a pixel composedof a photodiode and a transistor and a semiconductor substrate with thepixel formed thereon, wherein a pn junction portion formed between highconcentration regions of the photodiode is formed within thesemiconductor substrate and a part of the pn junction portion of thephotodiode is extended to a lower portion of the transistor formed onthe surface of the semiconductor substrate.

In the solid-state image pickup device according to the presentinvention, the pixel includes a charge readout transistor for readingout signal electric charges from the photodiode, a channel portion ofthe charge readout transistor being formed in the depth direction of thesurface of the semiconductor substrate.

In the solid-state image pickup device according to the presentinvention, the charge readout transistor includes a gate electrode and agate insulating film, the bottom portions of which are formed at theposition deeper than the depth of the pn junction portion of thephotodiode.

In the solid-state image pickup device according to the presentinvention, the charge readout transistor and the photodiode include agate electrode corresponding to their connected portion, the gateelectrode being located at the central portion of the photodiode.

In the solid state image pickup device according to the presentinvention, the charge readout transistor includes second conductivitytype source-drain regions, one of the second conductivity typesource-drain regions serving as a second conductivity type semiconductorregion comprising the photodiode as well.

In the solid-state image pickup device according to the presentinvention, the charge readout transistor has an effective channel lengthdetermined by a distance between the second conductivity typesemiconductor region comprising the photodiode formed by ionimplantation and the other second conductivity type source-drain regionformed on the surface of the semiconductor substrate of the chargereadout transistor.

In the solid-state image pickup device according to the presentinvention, the charge readout transistor includes a first conductivitytype semiconductor region formed between the second conductivity typesemiconductor region comprising the photodiode and a gate insulatingfilm in the portion corresponding to a peripheral portion of the gateelectrode of the charge readout transistor or a bottom portion of thegate electrode.

Further, in the solid-state image pickup device according to the presentinvention, the first conductivity type semiconductor region comprisingthe photodiode and the gate insulating film of the charge readouttransistor have formed therebetween a first conductivity type or secondconductivity type semiconductor region of which concentration is lowerthan that of the high concentration semiconductor region.

Furthermore, in the solid-state image pickup device according to thepresent invention, the semiconductor substrate introduces light into thephotodiode from its back.

According to the solid-state image pickup device of the presentinvention, since the pn junction formed between the high concentrationregions of the photodiode is provided in the inside of the semiconductorsubstrate such that a part of the pn junction may be extended to thelower portion of the transistor formed on the surface of thesemiconductor substrate, even when the pixel area is reduced, the largearea of the photodiode can be maintained. Accordingly, it is possible tomicrominiaturize the pixel size without lowering the saturated electriccharge amount (Qs) and sensitivity.

In the pixel, the transistor and the photodiode can be formed so as tooverlap with each other in the upper and lower direction in athree-dimensional fashion by forming the channel portion of the electriccharge readout transistor in the depth direction from the surface of thesemiconductor substrate. Thus, it becomes possible to realize themicrominiaturization of the pixel size while the area of the photodiodecan be increased.

The bottom portions of the gate electrode and the gate insulating filmof the electric charge readout transistor are formed at the positionsdeeper than the depth of the pn junction of the photodiode, whereby thechannel portion can be reliably formed between the photodiode and thesource-drain region. Thus, it is possible to make the operations of theelectric charge readout transistor become reliable.

The gate electrode of the electric charge readout transistor is formedat the position of the central portion of the photodiode, whereby adistance from the periphery of the photodiode and a distance from thegate electrode can be made equal to each other. Thus, it is possible totransfer all signal electric charges from the photodiode to the electriccharge readout transistor efficiently without remaining electriccharges.

Since one second conductivity type source-drain region of the electriccharge readout transistor serves as the second conductivity typesemiconductor region comprising the photodiode as well, the effectivechannel length of the electric charge readout transistor can bedetermined.

Since the effective channel length of the electric charge readouttransistor is determined based upon the length between the secondconductivity type semiconductor region formed by ion implantation andthe other second conductivity type source-drain region formed on thesurface of the substrate of the electric charge readout transistor, evenwhen some dispersions are produced in the position of the bottom portionof the gate electrode of the electric charge readout transistor, theeffective channel length can be prevented from being fluctuated andhence it is possible to provide a highly-reliable solid-state imagepickup device.

Since the first conductivity type semiconductor region is providedbetween the second conductivity type semiconductor region and the gateinsulating film of the photodiode at the portion corresponding to theperipheral portion of the gate electrode of the electric charge readouttransistor and the bottom portion of the gate electrode, it is possibleto suppress the occurrence of a leakage current due to a defect ofphotodiode.

Since the first conductivity or second conductivity type semiconductorregion with a concentration lower than that of the high concentrationsemiconductor region is formed between the first conductivity type highconcentration semiconductor region and the gate insulating film of thephotodiode, signal electric charges can be easily transferred in theelectric charge readout transistor while the electric chargeaccumulation amount of the photodiode is being kept.

Since light is introduced from the back surface of the semiconductorregion into the photodiode, while the area of the photodiode is beingincreased, it is possible to microminiaturize the pixel size withoutlowering the saturated electric charge amount and the sensitivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing a main portion of asolid-state image pickup device according to the related art;

FIG. 2 is a circuit diagram showing an equivalent circuit of a unitpixel of a solid-state image pickup device according to an embodiment ofthe present invention;

FIG. 3 is a schematic cross-sectional view showing a solid-state imagepickup device according to an embodiment of the present invention;

FIG. 4 is a partly cross-sectional view showing a main portion of asolid-state image pickup device according to another embodiment of thepresent invention; and

FIGS. 5A to 5F are respectively process diagrams showing a method ofmanufacturing a solid-state image pickup device according to theembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described below with reference to thedrawings.

FIG. 2 is a schematic circuit diagram showing an equivalent circuit of aunit pixel that can be applied to a solid-state image pickup device,that is, a CMOS type solid-state image device according to an embodimentof the present invention.

A unit pixel, generally depicted by reference numeral 20 in FIG. 2, ofthis CMOS type solid-state image pickup device is composed of aphotodiode 3, four MOS transistors, that is, an electric charge readouttransistor 4, a reset transistor 5, an amplifying transistor 6 and avertical selection transistor 7. The photodiode 3 is connected to onemain electrode of the electric charge readout transistor 4 and the othermain electrode of the electric charge readout transistor 4 is connectedto one main electrode of the reset transistor 5. The other mainelectrode of the reset transistor 5 is connected to one main electrodeof the amplifying transistor 6 and the other main electrode of theamplifying transistor 6 is connected to one main electrode of thevertical selection transistor 7.

An FD (floating diffusion) region corresponding to a connection pointbetween the electric charge readout transistor 4 and the resettransistor 5 is connected to the gate electrode of the amplifyingtransistor 6. A connection point between the reset transistor 5 and theamplifying transistor 6 is connected to a power supply line 8 led outfrom a voltage source Vdd. Further, the other main electrode of thevertical selection transistor 7 is connected to a vertical signal line9. A horizontal selection transistor 17 is connected between thevertical signal line 9 and a horizontal signal line (not shown).

Then, a vertical readout pulse φ TG is applied to the gate electrode ofthe electric charge readout transistor 4, a reset pulse φ R is appliedto the gate electrode of the reset transistor 5 and a vertical selectionpulse φ SEL is applied to the gate electrode of the vertical selectiontransistor 7.

A large number of unit pixels 20 are arranged in a two-dimensionalmatrix fashion (in an XY matrix fashion) to thereby constitute a CMOStype solid-state image pickup device.

In this unit pixel 20, signal electric charges are accumulated in thephotodiode 4 by photoelectric conversion. The vertical readout pulse φTG is applied to the gate electrode of the electric charge readouttransistor 4 to conduct the electric readout transistor 4 and signalelectric charges of the photodiode 3 are transferred to the FD region tochange the electric potential in the FD region. A signal voltage isapplied from the FD region to the gate electrode of the amplifyingtransistor 6 and it is converted into a signal electric current by theamplifying transistor 6. On the other hand, the vertical selection pulseφ SEL is applied to the gate electrode of the vertical selectiontransistor 7 to conduct the vertical selection transistor 7 so that thesignal electric current appears on the vertical signal line 9. Thissignal electric current is supplied through the horizontal selectiontransistor 17 to the horizontal signal line (not shown) and outputted tothe outside from an output unit (not shown) in response to a verticalselection pulse (not shown).

A solid-state image pickup device, that is, a CMOS type solid-stateimage pickup device according to an embodiment of the present inventionwill be described next with reference to FIG. 3. In this embodiment,while the present invention is applied to the CMOS type solid-stateimage pickup device including the unit pixel 20 composed of onephotodiode and four transistors shown in FIG. 2, the present inventionis not limited thereto and can also be applied to a CMOS typesolid-state image pickup device including other unit pixel composed ofMOS transistors the number of which is different from that of theabove-mentioned transistors comprising the unit pixel 20.

FIG. 3 is a schematic cross-sectional view showing a main portion of apixel, accordingly, the unit pixel composed of the photodiode 3 and thethree transistors of the electric charge readout transistor 4, the resettransistor 5 and the amplifying transistor 6.

In the CMOS type solid-state image pickup device 1 according to thisembodiment, a pixel separation region 25 for separating each pixel isformed on the surface of a first conductivity type, for example, a ptype silicon semiconductor substrate 2 and the photodiode 3 and aplurality of MS transistors, in this embodiment, four MS transistors ofthe electric charge readout transistor 4, the reset transistor 5, theamplifying transistor 6 and the vertical selection transistor (notshown) are formed on each pixel separation region 25 and thereby theunit pixel 20 is formed. A large number of unit pixels 20 are arrangedin a two-dimensional matrix fashion. The pixel separation region 25 isformed of a field insulating film (SiO₂) film.

According to this embodiment, in particular, a plurality of MStransistors, that is, the electric charge readout transistor 4, thereset transistor 5, the amplifying transistor 6 and the verticalselection transistor (not shown) are formed on the surface of thesemiconductor substrate 2, and the photodiode 3 is formed on the insideof the semiconductor substrate 2 in such a manner that it may be locatedunder the electric charge readout transistor 4, the reset transistor 5,the amplifying transistor 6 and the vertical selection transistor (notshown). At that time, the channel portion of the electric charge readouttransistor 4 for reading out signal electric charges from the photodiode3 is formed in the depth direction, preferably, in the verticaldirection relative to the surface of the semiconductor substrate 2.

As shown in FIG. 3, the photodiode 3 is composed of a p typesemiconductor region (p⁺ region) 12 with a high impurity concentrationformed on the surface side of the semiconductor substrate 2 and an ntype semiconductor region 11 composed of a high concentration impurityregion (n⁺ region) 11A and a low impurity concentration region (nregion) 11B adjoining the p type semiconductor region 12 and which areformed in the depth direction toward the back side of the semiconductorsubstrate 2. A principal pn junction j lies within the semiconductorsubstrate 2 and it is formed in such a manner that a part of the pnjunction may be extended to the lower portion of the MOS transistor. Inthis case, as is clear from FIG. 3, the photodiode 3 is formed acrossthe regions of the adjacent unit pixels 20 separated by the pixelseparation region 25 when seen from the surface side of thesemiconductor substrate 2. When seen from the back side of thesemiconductor substrate 2, the region of the photodiode 3 corresponds tothe region of the unit pixel 20.

In the electric charge readout transistor 4, a high impurityconcentration n type semiconductor region, that is, so-called n⁺source-drain region 14 is formed on the surface of the semiconductorsubstrate 2. Also, a groove portion 18 is formed on the photodiode 3,for example, at the position deeper than the pn junction j from thesurface of the semiconductor substrate 2 in such a manner that it mayadjoin the n⁺ source-drain region 14. In other words, the groove portion18 is formed in the depth direction reaching the inside of the n⁺ region11A through the pn junction j, preferably, the groove portion 18 in thevertical direction should be formed relative to the substrate surface. Agate insulating film (for example, silicon oxide film, etc.) 10 isformed on the inner wall of this groove portion 18 across the n⁺source-drain region 14 and the n⁺ region 11A of the photodiode 3, and acolumnar gate electrode 19 is formed on the gate insulating film 10 soas to bury the groove portion 18. As a result, the channel portion 21 ofthe electric charge readout transistor 4 is formed in the depthdirection, preferably, in the vertical direction of the surface of thesemiconductor substrate 2 in correspondence with the inner wall of thegroove portion 18. The bottom portions of the gate electrode 19 and thegate insulating film 10 may be formed in correspondence with the depthposition of the pn junction j of the photodiode 3.

Further, in the portion corresponding to the lower peripheral portioncontaining the bottom portion of the gate electrode 19 of the electriccharge readout transistor 4, a p type semiconductor region (p⁻ region)with an impurity concentration lower than that of the p⁺ semiconductorregion 12 or an n type semiconductor region (n⁻ region) 13 a, in thisembodiment, a p⁻ region is formed between the n⁺ region 11A constructingthe photodiode 3 and the gate insulating film 10. Also, in the p typesemiconductor region (p⁺ region) 12 with a high impurity concentrationcomprising the photodiode 3, a portion 13 b near the gate insulatingfilm 10 is formed as a p type region (p⁻ region) with a low impurityconcentration. In this case, the p⁻ region 13 a and the p⁻ region 13 bcan be formed with the same impurity concentration.

Then, one n⁺ source-drain region 14, the n⁺ region 11A of the photodiode3 and which serves as the other n⁺ source-drain region as well and thegate electrode 19 constitute the electric charge readout transistor 4.This n⁺ source-drain region 14 becomes the FD (floating diffusion)region.

All signal electric charges photoelectrically converted and accumulatedby the photodiode 3 should be read out from the photodiode 3 to theelectric charge readout transistor 4 with high efficiency. To this end,it is desirable that the gate portion of the electric charge readouttransistor 4 corresponding to the connected portion between the electriccharge readout transistor 4 and the photodiode 3, that is, the gateelectrode 19 should be formed at the central portion in which distancesfrom the peripheral portions of the photodiode 3 become substantiallythe same as shown in FIG. 3. In FIG. 3, the gate electrode 19 is formedso as to contact with the source-drain region 14 and the pixelseparation region 25 through the gate insulating film 10.

At the same time the above-described n⁺ source-drain region 14 isformed, n type semiconductor regions with a high impurity concentration,that is, n⁺ source-drain regions 15 and 16 are formed on the othersurface of the p type semiconductor substrate 2.

Then, the reset transistor 15 is comprised of the n⁺ source-drainregions 14 and 15 and a gate electrode 24 formed on the p typesemiconductor substrate 12 between the two n⁺ source-drain regions 14and 15 through the gate insulating film 10. Also, the amplifyingtransistor 16 is comprised of the n⁺ source-drain regions 15 and 16 andthe gate electrode 24 formed on the p type semiconductor substrate 2between the two n⁺ source-drain regions 15 and 16 through the gateinsulating film 10. Although not shown, the vertical selectiontransistor (see FIG. 2) also is comprised of a pair of source-drainregions and a gate electrode formed on the p type semiconductorsubstrate 2 between the pair of source-drain regions through the gateinsulating film 10 in a like manner.

Each of the above-mentioned gate electrodes 19, 23 and 24 can be formedof a polysilicon film, for example. Also, a connection conductor 24 andinterconnections 27, 28 and the like also can be formed of polysiliconfilms, for example.

A multilayer interconnection 27 containing the power supplyinterconnection 28 is formed through the interlayer insulator 26 on thesemiconductor substrate 2 on which there are formed the MOS transistors4 to 6, the vertical selection transistor and the like. Of themultilayer interconnection, the power supply interconnection 28 isconnected through a connection conductor 29 to the n⁺ source-drainregion 14 which serves as the FD (floating diffusion) region of eachpixel.

While the p type semiconductor region (p⁻ region) 13 a with a lowimpurity concentration is formed between the n⁺ region 11A comprisingthe photodiode 13 and the gate insulating film 10 in the portioncorresponding to the lower peripheral portion containing the bottomportion of the gate electrode 19 of the electric charge readouttransistor 4 in the above-mentioned embodiment, the present invention isnot limited thereto and the following variant is also possible. That is,as shown in FIG. 4 which is a schematic cross-sectional view showing amain portion of a solid-state image pickup device according to anotherembodiment of the present invention, a p type semiconductor region (p⁻region) 13 c with a low impurity concentration may be formed between then⁺ region 11A comprising the photodiode 3 and the gate insulating film10 only in the portion corresponding to the bottom portion of the gateelectrode 19 of the electric charge readout transistor 4.

In the above-mentioned CMOS type solid-state image pickup device 1,light L is introduced from the back of the semiconductor substrate 1into the CMOS type solid-state image pickup device 1 and this light L isreceived by the photodiode 3. Although not shown, the semiconductorsubstrate 2 includes a color filter provided at the back thereof andalso includes a suitable device such as an on-chip microlens at theposition corresponding to each pixel 20.

A method of manufacturing the above-mentioned CMOS type solid-stateimage pickup device 1, in particular, a method of manufacturing a mainportion containing the photodiode 3 and the electric charge readouttransistor 4 according to the embodiment of the present invention willbe described next with reference to FIGS. 5A to 5F.

First, as shown in FIG. 5A, the n type semiconductor substrate (n⁺region 11A and n region 11B) comprising the photodiode 3, the p typesemiconductor region, that is, the central p⁻ region 13 a and the p⁺regions 12 at both sides of the p⁻ region 13 are selectively depositedat the predetermined depth position of each pixel forming region of thep type semiconductor substrate 2 by ion implantation.

Next, as shown in FIG. 5B, the columnar groove portion 18 is formed inthe substrate depth direction, preferably, in the vertical direction soas to reach the inside of the n⁺ region 11A from the surface of thesemiconductor substrate 2 in correspondence with the central portion ofthe p⁻ region 13 b by selective etching, for example. Then, the gateinsulating film (for example, silicon oxide film) 10 is deposited on theinner wall surface of the columnar groove portion 18 and the surface ofthe substrate 2 by a CVD (chemical vapor deposition) method or a thermaloxidation method.

Next, as shown in FIG. 5C, the p⁻ region 13 a is formed on the n⁺ region11A of the photodiode 3 at its portion corresponding to the lowerperipheral portion containing the bottom portion of the groove 18 byimplanting ions of p type impurities with a low concentration from theoblique directions.

Next, as shown in FIG. 5D, a polysilicon film 31 is deposited on thewhole surface of the surface of the substrate 2 so as to bury the grooveportion 18 and a resist mask 32 is formed on the region in which thegate electrode is to be formed.

Next, as shown in FIG. 5E, the columnar gate electrode 19 buried intothe groove 18 is formed by patterning the polysilicon film 31 throughthe resist mask 32.

Next, as shown in FIG. 5F, the pixel separation region 25 and thesource-drain regions 14, 15 and 16 of the respective MOS transistors areformed by ion implantation. Then, the gate electrodes 23 and 24 formedof other polysilicon films are formed. The gate electrodes 19, 23, 24and the like can be formed at the same time.

Also, it is possible to form the columnar gate electrode 19 by formingthe groove portion 18 after the respective MOS transistors were formed.

According to this manufacturing method, since the photodiode 3 is formedby ion implantation before the electric charge readout transistor 4 isformed, even when dispersions occur in the process for forming thegroove portion 18, an effective gate length d can be determined easilyand accurately. That is, the effective gate length d is determined basedupon a distance between the n⁺ region 11A comprising the photodiode 3and the bottom portion of the n⁺ source-drain region 14 of the electriccharge readout transistor 4.

According to the CMOS type solid-state image pickup device of theabove-mentioned embodiments, since the photodiode 3 is located in thepixel region 20 in a three-dimensional fashion such that it may belocated under a plurality of MOS transistors formed on the substratesurface, for example, the electric charge readout transistor 4, thereset transistor 5, the amplifying transistor 6, the vertical transferregister (not shown) and the like, while the area of the photodiode 3 isbeing increased, the area of the pixel can be reduced. That is, sincethe area of the photodiode is increased and incident light is receivedfrom the back of the substrate, it is possible to microminiaturize thepixel size without lowering the saturated electric charge amount (Qs)and sensitivity.

Electric charges can be read out from the photodiode 3 by the electriccharge readout transistor 4 of which channel portion 21 is formed in thedepth direction, preferably, in the vertical direction relative to thesurface of the semiconductor substrate 2. Also, when the gate electrode19 of the electric charge readout transistor 4 is located at the centralportion of the photodiode 3, signal electric charges can be efficientlyread out from the whole region of the photodiode 3 through the channelportion 21 to the electric charge readout transistor 4. Accordingly, itbecomes easy to read out electric charges from the photodiode 3electrically.

Since the n⁺ source-drain region 14 of the electric charge readouttransistor 4 serves as the n⁺ region 11A comprising the photodiode 3 aswell, there can be determined the effective channel length d of theelectric charge readout transistor 4.

The bottom portions of the gate electrode 19 and the gate insulatingfilm 10 of the electric charge readout transistor 4 are formed at theposition identical to the depth of the pn junction j of the photodiode 3or at the position deeper than the pn junction j, whereby the channelportion 21 can be reliably formed between the photodiode 3 and the n⁺source-drain region 14 and the electric charge readout transistor 4 canbe operated reliably.

Also, since the n⁺ region 11A of the photodiode 3 serves as the other n⁺source-drain region of the electric charge readout transistor 4 as well,the bottom portion of the columnar gate electrode 19 of the electriccharge readout transistor 4 is extended to the position deeper than theposition of the pn junction j of the photodiode 3 and the electriccharge readout transistor 4 is formed after the photodiode 3 was formed,the effective gate length d of the electric charge readout transistor 4can be determined by the depth position of the n⁺ region 11A of thephotodiode 3.

More specifically, even when dispersions occur in the process forforming the groove portion 18, it is possible to determine the accurateeffective gate length d. Further, since the p⁻ region 13 a is formedbetween the gate insulating film 10 of the electric charge readouttransistor 4 and the n⁺ region 11A of the photodiode 3, it is possibleto suppress a leakage current from being produced due to defects of thephotodiode 3. Furthermore, since the p⁻ region 13 b is formed betweenthe gate insulating film 10 of the electric charge readout transistor 4and the p⁺ semiconductor region 12 of the photodiode 3, while theelectric charge accumulation capacity of the photodiode 3 is beingmaintained, it becomes possible to facilitate transfer of electriccharges in the electric charge readout transistor 4.

According to the solid-state image pickup device of the presentinvention, since the pn junction formed between the high concentrationregions of the photodiode is provided in the inside of the semiconductorsubstrate such that a part of the pn junction may be extended to thelower portion of the transistor formed on the surface of thesemiconductor substrate, even when the pixel area is reduced, the largearea of the photodiode can be maintained. Accordingly, it is possible tomicrominiaturize the pixel size without lowering the saturated electriccharge amount (Qs) and sensitivity.

In the pixel, the transistor and the photodiode can be formed so as tooverlap with each other in the upper and lower direction in athree-dimensional fashion by forming the channel portion of the electriccharge readout transistor in the depth direction from the surface of thesemiconductor substrate. Thus, it becomes possible to realize themicrominiaturization of the pixel size while the area of the photodiodecan be increased.

The bottom portions of the gate electrode and the gate insulating filmof the electric charge readout transistor are formed at the positionsdeeper than the depth of the pn junction of the photodiode, whereby thechannel portion can be reliably formed between the photodiode and thesource-drain region. Thus, it is possible to make the operations of theelectric charge readout transistor become reliable.

The gate electrode of the electric charge readout transistor is formedat the position of the central portion of the photodiode, whereby adistance from the periphery of the photodiode and a distance from thegate electrode can be made equal to each other. Thus, it is possible totransfer all signal electric charges from the photodiode to the electriccharge readout transistor efficiently without remaining electriccharges.

Since one second conductivity type source-drain region of the electriccharge readout transistor serves as the second conductivity typesemiconductor region comprising the photodiode as well, the effectivechannel length of the electric charge readout transistor can bedetermined.

Since the effective channel length of the electric charge readouttransistor is determined based upon the length between the secondconductivity type semiconductor region formed by ion implantation andthe other second conductivity type source-drain region formed on thesurface of the substrate of the electric charge readout transistor, evenwhen some dispersions are produced in the position of the bottom portionof the gate electrode of the electric charge readout transistor, theeffective channel length can be prevented from being fluctuated andhence it is possible to provide a highly-reliable solid-state imagepickup device.

Since the first conductivity type semiconductor region is providedbetween the second conductivity type semiconductor region and the gateinsulating film of the photodiode at the portion corresponding to theperipheral portion of the gate electrode of the electric charge readouttransistor and the bottom portion of the gate electrode, it is possibleto suppress the occurrence of a leakage current due to a defect ofphotodiode.

Since the first conductivity or second conductivity type semiconductorregion with a concentration lower than that of the high concentrationsemiconductor region is formed between the first conductivity type highconcentration semiconductor region and the gate insulating film of thephotodiode, signal electric charges can be easily transferred in theelectric charge readout transistor while the electric chargeaccumulation amount of the photodiode is being kept.

Since light is introduced from the back of the semiconductor region intothe photodiode, while the area of the photodiode is being increased, itis possible to microminiaturize the pixel size without lowering thesaturated electric charge amount and the sensitivity.

Having described preferred embodiments of the invention with referenceto the accompanying drawings, it is to be understood that the inventionis not limited to those precise embodiments and that various changes andmodifications could be effected therein by one skilled in the artwithout departing from the spirit or scope of the invention as definedin the appended claims.

1. A solid-state image pickup device comprising: a pixel comprised of a photodiode and a transistor; and a semiconductor substrate with said pixel, wherein a pn junction portion formed between high concentration impurity regions of said photodiode is formed within said semiconductor substrate and a part of said pn junction portion of said photodiode is extended to a lower portion of said transistor formed at the surface of said semiconductor substrate, and wherein the pn junction of the the photodiode is formed across regions of adjacent unit pixels with a pixel separation region therebetween.
 2. A solid-state image pickup device comprising: a pixel comprised of a photodiode and a transistor; and a semiconductor substrate with said pixel, wherein a pn junction portion formed between high concentration impurity regions of said photodiode is formed within said semiconductor substrate and a part of said pn junction portion of said photodiode is extended to a lower portion of said transistor formed at the surface of said semiconductor substrate; wherein said pixel includes a charge readout transistor for reading out signal electric charges from said photodiode a channel portion of said charge readout transistor being formed in the depth direction of the surface of said semiconductor substrate.
 3. A solid-state image pickup device according to claim 2, wherein said charge readout transistor includes a gate electrode and a gate insulating film, the bottom portions of which are formed at the position deeper than the depth of said pn junction portion of said photodiode.
 4. A solid-state image pickup device according to claim 2, wherein said charge readout transistor includes a gate electrode, said gate electrode being located at the central portion of said photodiode.
 5. A solid state image pickup device according to claim 2, wherein said substrate is a first conductivity type and said charge readout transistor includes second conductivity type source-dram regions.
 6. A solid-state image pickup device according to claim 5, wherein said charge readout transistor has an effective channel length determined by a distance between said second conductivity typo semiconductor region comprising said photodiode formed by ion implantation and a second conductivity type source-drain region formed at the surface of said semiconductor substrate.
 7. A solid-state image pickup device according to claim 5, wherein said charge readout transistor includes a first conductivity type semiconductor region formed between said second conductivity type semiconductor region comprising said photodiode and a gate insulating film.
 8. A solid-state image pickup device according to claim 7, wherein said first conductivity type semiconductor region comprising said photodiode and said gate insulating film of said charge readout transistor have formed therebetween a first conductivity type or second conductivity type semiconductor region with an impurity concentration which is lower than that of said high concentration semiconductor region.
 9. A solid-state image pickup device according to claim 1, wherein said semiconductor substrate introduces light into said photodiode from its back. 